A diffraction objective lens having a diffraction structure formed on the optical surface has been put into commercial use as an objective lens for an optical pickup apparatus. In an objective lens for the optical pickup apparatus interchangeably used with both the DVD and CD, for example, the spherical aberration caused by the difference in the substrate thickness between the DVD and CD is offset by the difference in the diffraction action attributable to the difference in the wavelength used, whereby interchangeable use is ensured.
In recent years, the trend in the optical pickup apparatus is moving toward the use of shorter wavelengths for the laser light source employed as a light source for reproduction of information recorded on an optical information recording medium and for recording of the information on an optical information recording medium. The laser light source having a wavelength of 400 through 420 nm is coming into commercial use, as exemplified by the blue-violet semiconductor laser and the blue SHG laser and others wherein the wavelength of the infrared semiconductor laser is converted using the non-linear optical effect. Use of these blue-violet laser light sources allows the 15- through 20-GB information to be recorded on an optical information recording medium having a diameter of 12 cm, when using the objective lens having the same numerical aperture (NA) as that of the DVD (Digital Versatile Disk). If the NA of the objective lens is increased to 0.85, the 23- through 25-GB information can be recorded on the optical information recording medium with a diameter of 12 cm. In the following description of this Specification, the optical information recording medium using a blue-violet laser light source and the magneto-optical recording disk will be collectively called the “high-density optical information recording medium”.
In a high-density optical information recording medium using the objective lens having a NA of 0.85, there is an increase in the comatic aberration attributable to the skew of the optical information recording medium. Thus, in some of the recording media, the protective layer is designed thinner than that of the DVD (0.1 mm thick as compared to 0.6° mm for a DVD) to reduce the comatic aberration attributable to the skew. Incidentally, the mere function of appropriate information recording/reproduction using a high-density optical information recording medium of this type does not provide a sufficient value to the optical information recording medium player/recorder (optical information recording/reproduction apparatus). In view of the fact that the DVD and CD (compact disk) with a great variety of information recorded thereon is available on the market at present, the function of merely recording and reproducing using a high-density optical information recording medium is not sufficient. The effort for achieving the similar appropriate information recording/reproduction using the DVD and CD owned by the user can enhance the commercial value of the optical information recording medium player/recorder. Against this backdrop, the optical pickup apparatus mounted on the optical information recording medium player/recorder for high-density optical information recording medium is required to be capable of recording/reproducing the appropriate information, while maintaining compatibility with all of the high-density optical information recording mediums, DVD and CD. To achieve this, many proposals have been submitted regarding the objective lens for optical pickup apparatus interchangeably used with both the high-density optical information recording medium and DVD/CD. In one of such proposals, for example, the spherical aberration resulting from the difference of substrate thickness between the high-density optical information recording medium and DVD/CD is offset by the difference in the diffraction attributable to the difference in the wavelength to be used, whereby compatibility is ensured. Alternatively, without being restricted to interchangeable use, many other proposals have been made, including the diffraction type optical element for color correction and correction of the spherical aberration resulting from temperature change.
A blaze structure having a serrated cross section, stairway structure, binary structure and others have been known as the optical path difference providing structure in such a diffraction optical element. However, it doesn't necessarily follow that only one of these optical path difference providing structures is used with the optical pickup device. A plurality of types of optical path difference providing structures may be superimposed in the same region of one optical surface of an objective lens, for example, or a plurality of types of optical path difference providing structures may be provided in the different regions on one optical surface of the objective lens. Alternatively, both the collimator and objective lens may be provided with optical path difference providing structures, depending on each situation.
The diffraction efficiency of the diffraction optical element is determined by the optical path difference providing structure and wavelength to be utilized. However, diffraction efficiency tends to vary with reference to the design standard value (hereinafter referred to as “wavelength-dependent fluctuation of diffraction efficiency”) by a change in the wavelength to be used and a change in refractive index resulting from the change in the wavelength to be used. Especially when high-order diffraction light is used, the wavelength-dependent fluctuation of diffraction efficiency is greater than when low-order diffraction light is used. For example, in an interchangeable objective lens wherein the blue-violet laser light for high-density optical information recording medium, red laser for DVD and infrared laser for CD are used in common, high-order diffraction light (second- or higher-order) is used for the blue-violet laser light to perform aberration correction in many cases. Thus, in such cases, there is an increase in the wavelength-dependent fluctuation of diffraction efficiency.
However, in the commonly used optical pickup apparatus, the intensity of the light emitted from the semiconductor laser is monitored and feedback control is provided in such a way as to get the spot having strength suitable for information recording/reproduction. However, a change in the diffraction efficiency of the objective lens is not monitored. If there is a great fluctuation in this change, the spot having the strength suitable for information recording/reproduction may not be obtained. By contrast, in the optical pickup apparatus wherein a diffraction optical element is used, the arts disclosed in the following Patent Literatures are available to get the appropriate diffraction efficiency:    Patent Literature 1: Japanese Patent Application Publication No. 2001-93179    Patent Literature 2: Japanese Patent Application Publication No. Hei 10 (1998)-133104